Tips on reducing Polycount in Scene
Here is something you can try:


Select one object and add the decimate modifier. Do not apply it.
Select all objects EXCEPT for the one with the modifier.
Shift-select the modified object. It will now be the active object
    (bright yellow-orange outline).
Press Ctrl+L and choose "Modifiers" from the Make Links menu.


This should apply the modifier across all objects and keep the modifier editable. 

Alternatively, you could apply the modifier in the first step and in the fourth step select "Object Data". Keep in mind this links the objects so certain changes made to the active object will take effect across all the others. Scaling and other transforms done in Object Mode should not affect the others.

If you need to unlink object data the method in 2.8 is select one object in Object Mode and in the Object menu (top left of the viewport) select Object> Relations> Make Single User> Object &amp; Data (or whatever serves your purpose) and select All.

In 2.79 and earlier select an object, hit U, and choose Object &amp; Data or whatever.